Spatial light modulators (SLM), as the term is used herein, are optical masks having a plurality of small pixel areas that are individually and selectively switchable. SLMs are used to modulate a reading optical wavefront, causing the wavefront to be either transmitted through the SLM (i.e. a transmission mode device), or to be reflected from the SLM (i.e. a reflection mode device), with a polarization that is determined by the modulator. These optical masks are usually two-dimensional, and comprise a plurality of small picture elements (i.e. pixels or PELS) that may be arranged in a two or three dimensional matrix of rows and columns.
In an optically addressable SLM, a write beam(s), for example a visible laser beam(s), programs or activates the individual pixels of the SLM to subsequently rotate the polarization of a read beam(s), for example an infrared laser beam(s). The write beam(s) programs the SLM by activating individual photosensitive pixel areas of the SLM. That is, those modulator areas that are to be programmed to rotate the polarization of a read beam(s) are activated by a write beam(s). U.S. Pat. No. 4,538,884 is an example. In the device of this patent, a pair of glass plates 1a and 2a support a pair of transparent electrodes 2a and 2b. A photoconductive layer 9, which can be amorphous silicon, is supported on electrode layer 2b. A plurality of aluminum reflectors 8 are incorporated into a transparent insulating layer 7 and are supported on the surface of the photoconductive layer, with the reflectors directly adjacent the photoconductive layer. An apertured shading layer 5 of carbon or metal is carried on the transparent insulating layer, each apertures 6 facing one of the reflectors. The space intermediate transparent insulating layer 7 and transparent electrode 2a is occupied by a liquid crystal 3. Other light activated spatial light modulators are shown in U.S. Pat. Nos. 4,619,501 and 4,655,554 wherein activating light produces a surface charge pattern that modulates the index of refraction of an electro-optic crystal; U.S. Pat. No. 4,693,561 wherein the voltage applied to aluminum strips that are in contact with a liquid crystal operate to destroy the twist of the liquid crystal in the grating space between the aluminum strips, and wherein activating light operates on a photoconductive layer to reduce the potential in the grating spaces, whereby light modulation occurs; and U.S. Pat. No. 4,679,910 which discloses two liquid crystal cells in series, a photoconductive substrate being disposed between the cells, and a mirror being disposed after the second cell. Other patents that may be of interest are U.S. Pat. No. 3,855,579 which shows an optic relay having a multi-layer dielectric mirror in direct contact with an electro-optical material; U.S. Pat. No. 3,824,002 which describes a liquid crystal light valve wherein all electrically conductive elements are isolated from the liquid crystal, and as a result, photoactivation must be accomplished by using the principle of impedance matching; U.S. Pat. No. 4,368,386 which discloses a liquid crystal image converter that can be configured to operate in either the transmissive or the reflective mode; and U.S. Pat. No. 4,277,145 which discloses a reflective mode liquid crystal display device wherein a writing beam in the presence of an applied voltage causes a transition in the liquid crystal to the isotropic phase, whereupon an image may be readout by light that passes through the liquid crystal and is reflected from a reflecting electrode.
The use of amorphous silicon photoconductor means in a liquid crystal device is suggested in the article "Amorphous silicon photoconductor in a liquid crystal spatial light modulator", by Paul R. Ashley and Jack H. Davis, APPLIED OPTICS, Jan. 15, 1987, Vol. 26, No. 2, at pages 241-246.
The use of amorphous silicon photoconductor means and ferroelectric liquid crystal means in a liquid crystal device is suggested in the article "High-speed light valve using an amorphous silicon photosensor and ferroelectric liquid crystals", by N. Takahashi, H. Asada, M. Miyahara and S. Kurita, APPLIED PHYSICS LETTERS, Vol. 51, No. 16, Oct. 19, 1987.
The use of a ferroelectric liquid crystal in an SLM having reflective mode photodiode or photoconductive portions is suggested in THE PROCEEDINGS OF SPIE--THE INTERNATIONAL SOCIETY FOR OPTICAL ENGINEERING, Vol. 754, Jan. 13-15, 1987, at pages 207-212, incorporated herein by reference.